The invention discloses a testing transformer. The testing transformer comprises a bracket, an iron core group and a phase electromagnetic coupling assembly, wherein the iron core group comprises multiple iron cores; a pair of first insulation cushion blocks which are arranged in the length direction of the transformer and a pair of second insulation cushion blocks which are arranged in the width direction of the transformer are arranged in the middle part of each iron core; the pair of the first insulation cushion blocks and the pair of the second insulation cushion blocks are crossed to form a cross-shaped insulating group; the phase electromagnetic coupling assembly comprises a phase electromagnetic coupling iron core and four groups of lamination sheets; the phase electromagnetic coupling iron core is arranged in the center of the insulating group; the four groups of lamination sheets are arranged on the periphery of the phase electromagnetic coupling iron core and positioned in four quadrants isolated by the cross-shaped insulating group; the four groups of lamination sheets are fixed on a pair of insulating plates; the two ends of the pair of the insulating plates are movably connected with a pair of parallel fixed rods in parallel; and the insulating plates can move along the fixed rods. According to the testing transformer, the coupling between upper and lower windings can be changed by setting that the lamination sheets slide along the fixed rods along with the insulating plates, so that the testing transformer is simple to operate and convenient to use in a test.